Apparatus for improving the lane resolution capabilities of omega navigation receivers

ABSTRACT

The invention consists of a system to improve the lane resolution of Omega Navigation Receivers. By measuring the average phase offset of the 3.4 kHz signal from the 10.2 kHz signal due to propagation variations, the offset can be added to the 3.4 kHz phase measurement to provide spatial coincidence of the 10.2 and 13.6 kHz lanes and thus provide better lane resolution of the 10.2 kHz lane.

Unite States Williams [72] Inventor: Melvin F. Williams, Washington, DC.

[73] Assignee: The United States of America as represented by theSecretary of the Navy 221 Filed: Feb.27, 1970 21 Appl.No.: 14,913

3,564,425 2/1971 Brok ..328/155 X Primary Examiner-Carl D. QuarforthAssistant Examiner-J. M. Potenza AttomeyR. S. Sciascia, Arthur L.Brannings and J. G. Murray [57] ABSTRACT The invention consists of asystem to improve the lane resolution of Omega Navigation Receivers. Bymeasuring the average phase offset of the 3.4 kHz signal from the 10.2kHz 52 US. Cl ..343/105 328/155 Sign due PmPagafio" can be added 5111111. C1 ..G0ls 1/30 the KHZ P measurement Pmide Spatial [58] Field ofSearch ..328/l55; 343 105 Cider! 9f the and KHZ lanes and thus providebetter lane resolution of the 10.2 kHz lane. [56] References Cited 4 cl4 Drawing Figures UNITED STATES PATENTS 3,493,971 2/1970 Earp ..343/105R DIURNAL SHIFT 10 2 KHZ PHASE 10.2 KHZ LOP SHIFTER DISPLAY 14 3.4 KM!12 1% LOP DISPLAY 3;: 35;; Or asm,

DIURNAL SHIFT APPARATUS F OR IMPROVING THE LANE RESOLUTION CAPABILITIESOF OMEGA NAVIGATION RECEIVERS STATEMENT OF GOVERNMENT INTEREST Theinvention described herein may be manufactured and used by or for theGovernment of the United States for governmental purposes without thepayment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION In the field of long range radio navigationsystems, it has been the general practice to employ receiving apparatusfor determining position either by use of hyperbolic or circularcoordinates.

One such long range radio navigation system with which this inventionmay be utilized is the Omega System. The operation of this system isdescribed in report number AD-63900 by Pierce, Palmer, Watt andWoodward, available from the Defense Documentation Center, entitledOmega a World Wide Navigational System.

The Omega System is a vlf to 14 kHz) radio navigation system which willprovide complete worldwide coverage with only eight transmittingstations. Omega, which provides position fixing with moderate (1 mile)accuracy, is usable by aircraft, Ships, land vehicles, and submarines atmoderate antenna depths. The system fills a requirement which has notbeen met by any preceding navigation system; celestial navigation is notall-weather; present electronic systems cannot provide global coverage;and inertial systems are expensive, limited in accuracy, and degradewith time.

Each of eight transmitting stations transmits a 10.2 kHz signal forapproximately one second in turn. Each transmitter is synchronized withall other transmitters; that is, all antenna currents are in absolutephase regardless of the transmitters location. This is accomplished bythe use of four atomic frequency standards at each transmitter location.The cesium (primary) frequency standards will be connected throughcombiner circuits to provide an output at the mean frequency of the fourstandards. The resulting output is applied to a frequency synthesizerwhich produces all frequencies required for the Omega signal format. Inthis manner the timing error between stations is reduced to onemicrosecond or less.

Since the transmissions are phase-locked, the difference signalfield-phase is everywhere stationary. The relative phase angle of aparticular pair of signals (from any two transmitting stations) at anygiven point depends solely upon the difference between the distance toone of the stations and the distance from the other. Furthermore, thesame phase angle will be observed at all points which have the samedifferences in distance from the stations. (The matter of resolving theresulting ambiguities will be discussed later.) The loci of such pointsform a contour of constant phase (isophase contour: fixed on the surfacewith respect to the locations of the corresponding pair oftransmitters). The relative radio frequency phase of every pair ofsignals observed at any point on the earth defines a known isophasecontour which contains that point. The intersection of two such contoursestablished by different pairs of stations defines the location of apoint.

The navigator can determine the line of position generated by anyconvenient pair of stations and then cross it with one or more linesderived from another pair or pairs of stations to obtain a position fix.The navigator will be able to choose position lines by Omega much as hechooses celestial lines of position for greatest accuracy and for largecrossing angles. He may make readings on four or five lines of position,but usually will choose the two pairs that jointly give the greatestprecision at his particular location.

The fundamental measurement performed by the Omega receiver is thedetermination of the relative phase of the 10.2 kHz signals from atleast two pairs of stations. This measurement establishes two or morelines of position and thus, a position fix. The receiver may be manualor partly or fully automatic, the degree of automation required beingdetermined by the type of vehicle in which the receiver is employed. Toobtain a fix with a manual receiver, the operator adjusts the receiverto read the lines of position he has chosen. Thereafter, the operationof the receiver will be automatic in the tracking of these signals,until the operator modifies his choice of pairs, or until arrival at hisdestination. The indication of position lines is continuous and may berecorded for the convenience of the navigator.

At 10 second intervals, each of the Omega stations transmits forapproximately 1 second at the basic frequency of 10.2 kHz. When observedat any given point, the combined transmissions of all eight stationsform a repeating 10 second cycle composed of the 10.2 kHz segments. Eachsegment of the cycle will be of a different amplitude and phase becauseof the differences in distances to the transmitting stations. Aspreviously mentioned, the radio frequency phases of all transmissionsare phase-locked, which causes the relative phases of all signalcomponents to be everywhere stationary. This establishes a fixed patternof a family of isophase contours which define positions in hyperboliccoordinates. On the base line between stations, the ambiguous hyperboliclane width is 98 microseconds 8 miles).

The stations always transmit in the same order with the length oftransmission varying from 0.9 to LI and to 1.2 seconds. The identity ofthe transmissions of a particular station is established by the time ofoccurrence in the sequence. An alternative method of stationidentification can be established by reference to Standard Time.

The 10.2 kHz portion of the Omega signal format forms a completeradio-location signal by which position can be defined in respect to thebasic position contour pattern. The relative phase angles of the 10.2kHz signals of each pair of stations define a family of contour patternsequal in number to twice the distance between stations in wavelengthsand spaced one-half wavelength (8 miles) apart on the base line. Onlyone contour of the family contains the position of the observer, and ameans must be provided whereby the 8 mile ambiguity can be resolved.

Resolution of the lane ambiguity is the process of selecting, from amongall the lanes in which the observer might be located, the particularlane that does contain his position. Thus, lane identification consistsessentially of establishing the position of the observer by independentmeans, to within a one-half lane.

In order to resolve the lane ambiguity, additional signals aretransmitted by the Omega stations. Each station of the eight transmits al3.6-kHz signal, one segment later in the multiplex sequence. The13.6-kHz transmissions are phase-locked to Standard Time and hence tothe 10.2-kHz signals. The combined transmissions of the eight stationsestablish a second pattern of isophase contours with three quarters ofthe lane width of the 10.2-kHz pattern. Since both patterns are familiesof hyperbolae about the same points, the two patterns are everywhereparallel. The combination of the two patterns forms broad lanes with 3times the width of the basic 10.2-k Hz pattern (24 miles on its baseline).

The length-coding pattern of the 13.6-kHz transmissions is identicalwith the coding pattern of the 10.2-kHz signals. This allows the samemultiplex timing to be used in the receiver to separate signals ineither channel. Since each station transmits its two frequencies inadjacent segments of the sequence, the two transmissions of each stationare always of different durations. In a manual visual type receiver, theadditional transmissions at 13.6-kHz provide the first stage of laneidentification. The 13.6-kHz signals are utilized by retuning thereceiver and observing the phase relationships of the signals occurringone place later in the multiplex sequence than those observed atlO.2-kI-Iz. The differences of the corresponding indications at the twofrequencies indicate which of the three lanes in a broad lane containsthe observer. If the difference is less than one-third cycle, theobserver is in the first lane of the three; if between one-third andtwo-thirds, the observer is in the middle lane, and if betweentwo-thirds and one cycle, the observer is in the third lane.

The unambiguous lane width on a base line is thus expanded from the 8miles of the basic 10.2-kHz phase contour pattern to 24 miles. Furtherexpansion of the unambiguous lane width can be obtained by additionaltransmissions at other related frequencies. For example, a third set oftransmissions at a frequency of 11.333 kHz defines a pattern of isophasecontours with a spacing of nine-tenths of basic 10.2-kHz pattern. Thereis then a triple coincidence every nine 10.2 kHz lanes, every ten 11.333kHz lanes and every 12 13.6 kHz lanes, extending the unambiguous lanewidth on the base line to 72 miles.

The difference between the relative phase at 10.2 kHz and 13.6 kHz (3.4kHz) provides contours with a lane width of 24 miles, the differencebetween the 10.2 kHz and 11.333 kHz signals (1.133 kHz)supplies a lanewidth of 72 miles.

The lane accuracy of Omega can be considered in two parts. One is theeffect of natural random fluctuations in the times of propagation of theradio signals and the other is the actual average velocity ofpropagation. Charts, tables, or both are provided to relate the daytimeOmega readings to the geographic position. Because there are minordiurnal and annual changes in the velocity of propagation, the navigatorwill also be given compensation graphs or tables that permit him toreduce his observed readings to equivalent daytime readings before heconsults his main chart or table. This information can be stored in acomputer or computed as required.

The successful resolution of lane ambiguities by phase differencemeasurements at several frequencies depends on the stability andpredictability of ionospheric propagation at the frequencies involved.For reliable lane identification the uncertainty in predictability mustbe consistently less than half a fine lane width at all seasons andtimes and at all ranges from the transmitting stations within therecommended coverage area. Several factors including velocitydispersion, diurnal and seasonal propagation changes and the ratiosbetween the difference frequency and fine lane frequency determine thereliability of lane identification.

The lane resolution capabilities of the Omega navigation system are notadequate at all times and at all places primarily because of theinability to predict propagation variations to the required accuracy. Ithas been found that the 13.6 kHz signal is not always affected by theatmosphere to the same degree as the 10.2 kHz signal, that is, there isa phase offset between the two signals. Thus, if there is a signal loss,upon reacquisition of the signal the navigator might incorrectlydetermine his 10.2 kHz lane, resulting in an 8 mile error. Thus, if the3.4 kHz phase offset results in an error greater than 4 miles (one halflane) the lane count will be incorrect.

SUMMARY OF THE INVENTION Accordingly, the general purpose of thisinvention is to provide a radio navigation receiver which embraces allthe advantages of similarly employed aircraft receivers and possessesnone of the aforedescribed disadvantages. To obtain this, the presentinvention contemplates a unique system of measuring the average phaseoffset between the 10.2 kHz Omega signal and the 3.4 kHz signal. Thisphase shift is stored and then used to modify the 3.4 kHz signal phaseafter a lost signal is reacquired.

OBJECTS OF THE INVENTION It is therefore an object of the presentinvention to provide a system for identifying the correct lane in ahyperbolic continuous wave radio position finding system.

Another object of the present invention is to provide a long rangenavigation system capable of correcting for propagation variations oftransmitted signals.

A still further object of the present invention is to rezero the broadlanes with the fine lanes in the Omega navigation system.

Yet another object of the present invention is to provide a radionavigation receiving system which operates automatically after theinterruption of signals to indicate the correct lane and thecorresponding position ofthe aircraft.

BRIEF DESCRIPTION OF THE DRAWING Other objects and features of theinvention will become apparent to those skilled in the art as thedisclosure is made in the following description of the invention asillustrated in the accompanying sheet of drawing in which:

FIGS. 1a, 1b and 1c are a diagram of the relationship between thevarious lanes utilized in the Omega navigation system.

FIG. 2 is a block diagram of the significant portions of the receivercircuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown how the Omega lane resolution operates. FIG. 1c is the 1,133 Hzlane zone derived by subtracting the 11.333 kHz signal and 10.2 kHzsignal. By reading the phase angle m of this signal, having a 72 milelane width, the observer can determine in which 3.4 kHz lane theaircraft is located. The 3.4 kHz lane, as shown in FIG. lb, has a 24mile lane width and is derived from the 13.6 kHz signal and the 10.2 kHzsignal. From this determination n, the exact 10.2 kHz lane location p asshown in FIG. la, having an 8 mile lane width, can be located. Thus thismethod utilizes the wide lane to resolve ambiguity and the narrow, 10.2kHz lane to retain maximum accuracy. In theory the zero phase lines ofall three signals should coincide at the same point as shown in FIG. 1.In practice, however, due to propagation anomalies which are a functionof area and times of day, the zero phase lines are shifted. Thus, if the3.4 kHz signal phase is offset with respect to the 10.2 kHz signal by anangle resulting in a 4 mile or larger difference between the two zerophase signals, an incorrect 10.2 kHz lane will be identified. It istherefore necessary to correct for this relative phase offset betweenthe two signals in order to enhance the Omega lane resolutioncapabilities. This correction is especially critical after periods ofsignal loss.

Referring now to FIG. 2, there is shown a system to correct for thisphase error. After being detected by an antenna, receiver, phasedetectors and tracking servos, (not shown), the 10.2 kHz line ofposition (LOP) phase difference signal and the 13.6 kHz LOP phasedifference signal both RF signals are fed to phase shifters 11 and 12,respectively, where the predicted diurnal phase corrections are added tothe two signals. The two output signals from phase shifters l1 and 12are fed to frequency differential 13, which may be a conventional sinewave superhetrodyne mixer whose output is the 3.4 kHz LOP. The 10.2 kHzLOP is displayed and is also fed to the phase differential 15, which maycomprise zero crossing detectors coupled to a multivibrator or any othermeans for measuring the phase difference between two signals. Phasecomparators may be found in the Manual of Classification of the PatentOffice Class 307, subclass 232 or Class 328, subclass 133 for examplewhere its phase is compared with the 3.4 kHz signal phase which has beenmultiplied by 3 by multiplier 14 which may be any conventionalmultiplier such as found at Page 547 of vol. 19 of the RadiationLaboratory Series of MIT, 1947. Ideally the zero phase of the twosignals should coincide, but if they differ, the phase difference isintegrated for a selected period of time and stored in integrator 16.The integrated phase offset signal stored in integrator 16 is added tothe 3.4 kHz LOP signal in adder 17 before being displayed. Should theOmega signals ever be lost, upon reacquisition this offset, added to the3.4 kHz signal, will improve coincidence of the zero phase lines of the10.2 and 13.4 kHz lanes. As is obvious, during signal loss, the Omegareceiver would operate in its dead reckoning mode as described in U.S.Pat. No. 3,388,397 issued June 11, 1968 to A. F. Thornhill and M. F.Williams.

Of course, two such block diagrams are required to obtain the two LOPsrequired for a fix. In addition, a similar block diagram will berequired to rezero the 1,133 Hz lane with respect to the 10.2 kHz lane.If a circular mode is utilized rather than the hyperbolic mode, the lanewidths would be doubled but the circuitry would remain the same. Theblock diagram disclosed above can be either mechanical, analog ordigital.

It can be seen that the present ratio navigation receiving systemencompasses the ability of self correction after a period of signalloss, and enhances the lane resolution of the Omega system.

While a particular embodiment of the invention has been illustrated anddescribed, it will be recognized that many modifications and changeswill occur to those skilled in the art and it is therefore contemplatedby the appended claims to cover such modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

l. A radio navigation system for eliminating phase offset between tworeceived signals due to environmental conditions comprising:

receiving means to receive predetermined signals;

a first signal and a second signal;

means for multiplying said first signal whereby it has the samefrequency as said second signal;

differential means for indicating the phase difference between saidmultiplied first signal and said second signal;

averaging and storing means connected to said differential means forcalculating and storing the average phase difference indicated by saiddifferential means; and

means connected to said storing means for varying the phase of saidunmultiplied first signal by said stored phase difference signal wherebysaid first signal and second signal are in phase.

2. A system as recited in claim 1 including means for deriving saidfirst signal by subtracting a third signal from said second signal.

3. A system as recited in claim 2, including phase shifters between saidreceiving means and said deriving means for inserting diurnal phasecorrections to said second and third signals.

4. A system as recited in claim 3, wherein said first signal is 3,400Hz, said second signal is 10,200 Hz, said third signal is 11,333 Hz,said three signals being included in the Omega navigation system.

1. A radio navigation system for eliminating phase offset between tworeceived signals due to environmental conditions comprising: receivingmeans to receive predetermined signals; a first signal and a secondsignal; means for multiplying said first signal whereby it has the samefrequency as said second signal; differential means for indicating thephase difference between said multiplied first signal and said secondsignal; averaging and storing means connected to said differential meansfor calculating and storing the average phase difference indicated bysaid differential means; and means connected to said storing means forvarying the phase of said unmultiplied first signal by said stored phasedifference signal whereby said first signal and second signal are inphase.
 2. A system as recited in claim 1 including means for derivingsaid first signal by subtracting a third signal from said second signal.3. A system as recited in claim 2, including phase shiftErs between saidreceiving means and said deriving means for inserting diurnal phasecorrections to said second and third signals.
 4. A system as recited inclaim 3, wherein said first signal is 3,400 Hz, said second signal is10,200 Hz, said third signal is 11,333 Hz, said three signals beingincluded in the Omega navigation system.